Oil Field Background
In hydrocarbon deposits, such as heavy oil and bitumen containing reservoirs, a significant fraction of the hydrocarbon resource is unrecoverable by current methods, including primary production and secondary water flood techniques. These existing recovery techniques access only a small portion of known heavy crude reserves, with the balance remaining trapped underground. This is particularly true for the heavier crudes and bitumens in the 10 to 22 degree API (American Petroleum Institute gravity) category, where the viscosity of the hydrocarbon deposits can range to several hundred poise, or greater. In particular, these heavier crudes are difficult to displace from the porous matrix (formation) toward a production well with water-based fluids due to viscous fingering (unstable liquid interface) and the unfavorable mobility ratio, making simple water flooding a non-viable option.
Cold Heavy Oil Production with Sand (CHOPS) technology has been used to recover heavy crudes from relatively thin unconsolidated oil sands. CHOPS is a non-thermal process where both sand and oil are produced simultaneously under primary conditions. The increased sand production leads to higher levels of oil production as well. Typical CHOPS recovery factors are only 5 to 15% of the original oil in place leaving significant reserves in place and major parts of the reservoir unproduced. As a result of producing sand from these reservoirs and formations, extremely high permeability pathways (“wormholes”) are generated in the oil producing formations. While it has been shown that in many cases these wormholes are open channels of up to several centimeters in diameter that can extend hundreds of meters into the formation, in other cases wormholes after primary oil production has ceased, may fill, collapse and/or regrow into a network of open channels and disturbed areas of high porosity and permeability. Wormholes can be a dynamic phenomenon. As sand production continues, the wormholes grow larger and longer extending deeper into the reservoir. Consequently, the existence of wormholes in a post-CHOPS oil-bearing formation makes application of most subsequent enhanced recovery techniques technically difficult, high risk, uneconomical or even impossible.
Following primary CHOPS production, additional flooding or other secondary techniques are often ineffective as water, or other fluids, and gases readily bypass the heavy oil as it follows the wormhole pathways within the post-CHOPS oil-bearing formation. Injected fluids then quickly breakthrough the formation during a flood because of the adverse mobility contrast between water and heavy oil.
Many techniques have been proposed and attempted for improving oil recovery efficiency, but large volumes of hydrocarbons remain in oil-rich formations even after secondary recovery efforts. One factor causing retention of the hydrocarbons in the formations is the inability to direct sufficient pressure on the hydrocarbon droplets residing in the pore spaces of the matrix oil formation to promote flow to the production well. Still, the increasing value of oil and increased demand has created a greater need for enhanced oil recovery methods to revitalize older wells, including those that have been abandoned due to a high ratio of water compared to the volume of total oil produced (water cut).
Enhanced oil recovery encourages the flow of previously trapped oil by effectively increasing the relative permeability of the oil embedded in the formation and by reducing the viscosity and surface tension of the oil. Numerous enhanced oil recovery technologies are currently practiced in the field including those involving thermodynamic, chemical, and mechanical processes. Heating the oil with steam often reduces the viscosity of the trapped oil, provided there is ready access to steam energy and heat losses can be managed. In CHOPS, steam is ineffective due to the oil zone being relatively thin and the heat transfer is too challenged.
Various EOR techniques have been explored to improve the recovery of oil after primary CHOPS production. Attempts have been made to utilize downhole steam generators to apply heat to post-CHOPS production in a flooding situation to reduce crude oil viscosity and to improve flow so the heavy oil can be driven to a (second) collection well. Microwave devices in horizontal CHOPS wells have been used to heat up the formation and to reduce viscosity to improve mobility of heavy oils. Further, microbial techniques have also been used to generate biogas, methane or CO2, to re-pressurize parts of a CHOPS well and thereby solubilize the gases into the heavy crude to lower viscosity and to provide drive energy for production. Solvent and solvent-steam injection EOR techniques has also been used to repressure post CHOPS reservoirs, lower viscosity, improve sweep and recovery. None of these techniques however provide a universal economical solution or have gained wide spread use. Other techniques are needed to add to the oil recovery tool box. A need exists, therefore, to have an effective way to recover oil from post-CHOPS oil-bearing formations.